Tells
I watch the player first, table second.
Initially, I might ask someone who has watched them play, or that knows of them, just how good they think the player is, where he is from, and how big they gamble.
The player's stance and form, where and how they grip the cue, then I watch their stroke real closely, and determine if they have any flaws in it.
By determining any flaws in their stroke, or if they hold the cue unconventionally, I can almost tell you if they will miss, and what type of shots that they will miss most often.
I then watch their habits around the table, how they inspect the table, where they look and when, preshot routine, and follow through. How they chalk a cue. You can tell if they feel comfortable moving around the table or whether it is weighing on them.
Next I evaluate their shot selection, and shape, whether they are just a good open shooter but can't bank worth beans, or whether they have the total game under control. How is there safety game is important also.
I also notice their carom and combination shots too.
I, then, and only then, will rate them, and determine if I think I could beat them or not with my normal game, not my best game.
They say 'The Devil is in the details', and that is true, because if you miss any of the details I outlined, it won't turn out like you expected it to.
One last thing - A good gambler (notice I said gambler, not player) never plays anyone without watching them play first. If you play guys cold, you are just asking to get beat.